greigite

Greigite is an iron sulfide mineral with the chemical formula Fe3S4. It is a ferrimagnetic mineral that crystallizes in the spinel structure and is sometimes described as iron(II,III) sulfide. In nature it occurs as fine-grained grains, frequently forming inclusions within sulfide-rich sediments.

Crystallography and magnetism: Greigite adopts an inverse spinel structure in which iron ions occupy tetrahedral and octahedral sites. It is ferrimagnetic at ambient temperatures, which gives rise to magnetic signals that can be preserved in sediments where it forms. This magnetic character makes greigite of interest to studies of paleomagnetism and biomagnetism.

Occurrence and formation: Greigite forms in reducing sedimentary environments that supply sulfide, and can develop diagenetically from other iron sulfides such as pyrrhotite and pyrite. It is commonly found in marine and freshwater sediments, bogs, and some hydrothermal settings, often as microscopic grains. Biomineralization by magnetotactic bacteria can also produce greigite.

Significance and applications: While not a major ore mineral, greigite is studied for its magnetic properties and potential roles in reconstructing ancient magnetic fields, as well as in nanomaterials research and investigations of biomineralization pathways.

Physical properties: Greigite is opaque with a metallic luster, typically black to brownish-black in color. It is relatively brittle and occurs as fine-grained material in natural samples.